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Homework Help: Voltage and Current lab report help

  1. Feb 25, 2009 #1
    I'm working on a basic physics lab report (not homework) and I'm trying to understand something better. We're establishing a relation between voltage and current using Ohms law. It goes as follows:

    We apply different voltages to two materials and record the current in order to determine the resistance. The two materials are a resistor and a light bulb. My problem has occured when graphing though.

    I know that the relationship for an ohmic material is linear, therefore it's simply (1/R). This would be true for the resistor. The lightbulb however is a nonohmic material, therefore it's not a linear relationship.

    The way my graph looks is not the same as in the book though. The book shows a graph which looks like an exponential function (x^2), approaches a verticle asymptote. My graph for the lightbulb though looks opposite (looks more like a sqrt function) and approaches a horizontal asymptote. When looking online I saw a V versus I graph for a filament and it looked the same as mine.

    My question is, why is this? Why do I have a different graph than what the book shows for a nonohmic material, but I see other "filament" graphs look the same as mine.

  2. jcsd
  3. Feb 25, 2009 #2
    Light bulbs have a low resistance at room temp. This resistance increases as much as 15 times at operating temperature. So the resistance is proportional to the applied voltage.
    From Wikipedia
    "The cold resistance of tungsten-filament lamps is about 1/15 the hot-filament resistance when the lamp is operating. For example, a 100-watt, 120-volt lamp has a resistance of 144 ohms when lit, but the cold resistance is much lower (about 9.5 ohms)"
  4. Feb 25, 2009 #3
    I understand that it's a nonlinear relationship because R=R0(1+[tex]\alpha[/tex](T-T0)). My confusion is why does the relationship differ for two, nonohmic materials. The book depicts a graph that shows a horizontal asymptote, yet my graph looks as though I have a vertical asymptote, as well does a graph from wikepedia for a filament depict the same thing. So why does one slope approach zero while the other slop approaches 1?
  5. Feb 25, 2009 #4
    You say 'nonohmic' yet the tungsten filament is absolutely ohmic. It's just different resistance at different temperatures, as are most ohmic resistors. Since it's designed to work at about 1000C the temp variation is large. Note that the typical coiled filament, when unrolled is about 20inches long.
    Varistors are non-ohmic. I suggest you google Wiki for a description.
    Last edited: Feb 25, 2009
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